Update: The server error I originally described no longer seems to happen.
However, it seems that when using the "Edit tag info" button to make and submit a tag wiki edit during the grace period for a previous edit, it no longer honors the grace period and instead always submits a new revision.
As an example, I just made an edit to the extended tag wiki for 10k-tools. Shortly after my first edit, I noticed a misspelling in the wiki, so I clicked the "Edit tag info" button again and made the fix, which went through. I then immediately remembered facing a server error in the past about this (the original report below), and decided to check the edit history to see if the error was indeed fixed. Instead, it seems to not have been fixed, and my next edit was recorded as a new revision.
Henry Ecker commented here that the grace period worked fine when clicking the "Edit" button on the latest revision in the edit history to just edit that portion of the tag info. I tested this out and confirmed that the grace period still worked in that case.
I still think there's an outstanding bug, though. In my opinion, the grace period should be fixed for the "Edit tag info" button so it properly makes grace period edits rather than creating new revisions, given that the editor form fills in the edit summary of the prior revision just like it does when the editor is opened within a question or answer's grace period. If it's an intentional design change that grace period edits aren't to be allowed anymore for tag wikis, then it should be consistent with the Edit button from the revision history, i.e., that should also create new revisions, and the edit summary should be blank like it is for new revisions of posts.
I recently submitted an edit to the tag wiki (not excerpt) for status-deferred. As I have trusted user privileges here on this site, my edit was binding.
I then noticed something else that needed editing in the wiki. I tried making that edit, which would have been incorporated into my initial edit as it was within five minutes of that, but I got an error message:
An error occurred submitting the post.
I had to wait five minutes after my original edit, and only then did it allow me to submit my edit, and it recorded it as a new revision (see its revision history).
When I initially tried again, I got another error message that I needed to wait 10 seconds before trying again:
You can perform this action again in 10 seconds.
It seems I was hitting the rate limit for tag wiki edits from privileged users, even though my edit wasn't being submitted. After waiting 10 seconds, I still got the first error above until five minutes transpired.
It seems there's some server-side issue when handling grace period edits by privileged users to tag wikis, plus attempted edits that were unsuccessful with this error are still tripping the rate limit (once every 10 seconds for privileged tag wiki edits). Can these please be fixed?